DNA damage is one of the most important factors in cancer development in humans, and yet it also contributes to the therapeutic efficacy of many anti-cancer drugs. Thus, a better understanding of the mechanistic basis underlying DNA double strand break (DSB) repair and cellular responses to DNA DSB inducing agents (i.e. ionizing radiation and chemotherapeutics) is essential in deciphering the molecular basis underlying cancer development, as well as providing a foundation for developing new anti-cancer strategies. Recent experiments have demonstrated that interplays between hMSH5 and various protein interacting partners are critically involved in DNA damage response and repair. In particular, the interaction between hMSH5 and hMSH4 provided a basis for the formation of a specific heterocomplex that was capable of binding to Holliday junction intermediates, implicating their potential functions in recombinational repair. Furthermore, hMSH5 physically and functionally interacted with c-Abl; the latter is a critical tyrosine kinase playing essential roles in cell cycle arrests, DNA repair, and apoptosis. Our recent preliminary results indicate that DSB triggers local recruitments of endogenous hMSH5 and hMSH4 proteins, and the DSB- induced hMSH5 loading is dependent on functional hMRE11 and hRad51 proteins. Our recent study reveals that the activation of c-Abl kinase can be modulated through hMSH5 interaction, in which this interaction promotes the activation of c-Abl kinase activity and leads to hMSH5 tyrosine phosphorylation. Together, our results suggest important roles of hMSH5 in mitotic DNA recombinational repair and cellular response to DNA damage, implicating that the function of hMSH5 in DNA repair is closely coupled with DNA damage response pathway. It is conceivable that any deviations of such dynamics could significantly affect normal cell cycle progression as well as cellular responses to radiomimetic cancer treatments. The overall objective of our studies outlined in this proposal is to decipher the molecular mechanisms underlying the functions of hMSH5 in recombinational repair and cellular response to DNA damaging agents through a series of systematic and comprehensive experimental explorations. The long- term goal of this research program is to elucidate the molecular mechanisms involved with different types of DNA recombinational repair and their links to DNA damage response pathways. The outcome of these studies will provide a foundation for developing more efficient therapeutic means and novel molecular targets in cancer treatments.